A military cargo plane carrying four people on a training run has crashed at an air force base in Alaska, the US Air Force has confirmed.

Witnesses reported seeing a ball of fire rising hundreds of feet high near downtown Anchorage. Access to the crash site at Elmendorf Air Force Base has been closed.

The C-17, from the 3rd Wing based at Elmendorf, had been carrying four people doing a training demonstration for an upcoming weekend air show, Lieutenant General Dana Atkins said.

Lt Gen Atkins said the plane was not an ejection aircraft and said no-one is expected to have survived. "It's likely there are fatalities involved in this mishap," he added.

Anchorage Fire Department captain Bryan Grella described seeing the crash from a downtown fire station about two miles away.

"It was a big, grey plume of smoke, and I saw a fireball go up," he said, adding that the plume went about 750ft in the air. A board of officers will investigate the crash.

This was the third aircraft incident in Anchorage this summer. In June, one child was killed and four others burned when a small plane crashed after taking off from the city's small-plane airport.

Days later, a small plane landed on the busy Glenn Highway, the only highway leading north out of Anchorage. There were no injuries.

Governor Sean Parnell and Senator Mark Begich issued statements expressing sadness over the crash and sending well-wishes to members of the military. "Alaskans are very connected to the military, and our thoughts and prayers are with Alaska's air force family," Mr Parnell said.